1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image formed on an image bearing member of an electrophotographic copying machine or a laser beam printer, into a toner image.
2. Description of the Related Art
In image forming apparatuses such as electrophotographic copying machines, developing devices conventionally applied to these image forming apparatuses, adopting a powder cloud method, a cascade method, a magnetic brush method are known. Among them, in a case of the magnetic brush method of a two-component development system, a two-component developer including and mixed with a magnetic carrier, a toner and the like is attracted to a magnetic field generating unit. Then, the developer is caused to ear up in a brush shape in magnetic pole portions, and an electrostatic latent image is developed by subjecting an electrostatic latent image on the photosensitive drum to friction, thereby image formation is carried out. In this case, since the magnetic carrier itself in the developer functions as a soft developing electrode, it is possible to cause the toner to adhere thereto in proportion to a charge density of the electrostatic latent image, in other words, it is suitable for reproduction of gradation image. In addition, the developing device itself has a feature that it can be configured in a small size.
As a magnetic brush developing device of the two-component development system, a magnetic brush development method employing a developing sleeve serving as a developer bearing member is generalized. To accomplish the purpose of efficiently developing the electrostatic latent image on the photosensitive drum, first, a two-component developer including a magnetic carrier made of powder of magnetic material such as, for example, ferrite, and a toner with pigment being dispersed into a resin, is agitated and mixed. Then, the toner is caused to possess an electric charge by frictional charging generated by friction with each other. On the other hand, the developing sleeve serving as a hollow cylindrical developer bearing member made of a nonmagnetic material having magnetic poles in the interior thereof is caused to retain the developer. The developer is conveyed from a developer container to a developing region facing the photosensitive drum, by the developing sleeve. In the developing region, by causing the developer to ear up by an action of the above-described magnetic field, and subjecting the developer to friction on the photosensitive drum surface, thereby the electrostatic latent image formed on the photosensitive drum is developed. The two-component magnetic brush development method employing the developing sleeve is used in many products, mainly in a monochrome digital copying machine or a full-color copying machine requiring a high image quality.
However, if a rotational movement speed of the photosensitive drum is made faster to respond to demand for speeding up to recent copying machines, development time is not sufficient with one developing sleeve, and as a result, preferred image formation may not be performed.
As a countermeasure against this case, there is a method for enhancing development efficiency by increasing a circumferential speed of the developing sleeve. However, when the circumferential speed of the developing sleeve is increased, a centrifugal force acting on the developer which forms the magnetic brush is increased, and scattering of the developer increases in amount. As a result, there is a negative effect, which could lead to causing contamination in the interior of the copying machine, and deteriorating the functions of the apparatus.
Therefore, as another countermeasure, as discussed in Japanese Patent Application Laid-Open No. 2004-21125, two, or three or more developer bearing members such as a developing sleeves are used and arranged to bring their circumferential surfaces into close proximity so that they adjoin each other. Then, a method for extending development time and enhancing development ability, by allowing the developer to be continuously conveyed traveling on each other's circumferential surface, what is called a multi-stage magnetic brush developing method is discussed.
Here, in the developing device provided with plural developing sleeves as described above, the developer may leak out from a spacing between an upstream developing sleeve and a downstream developing sleeve, in longitudinal end portions of the developing sleeves (non-developer-bearing region). A problem that the developer or toner aggregates leaks out of the spacing of such developing sleeves cannot be solved only by a technique for the past general end portion configuration (e.g., a technique for arranging magnetic members or magnet members in the developing sleeve end portions).
Thus, for example, Japanese Patent Application Laid-Open No. 2010-096922 discusses a configuration of providing a cover member, in the outside of developer bearing regions of an upstream developing sleeve and a downstream developing sleeve, on a path routed from a spacing between the upstream developing sleeve and the downstream developing sleeve toward the opposed image bearing member. It is configured such that the developer leaking from the spacing between the upstream developing sleeve and the downstream developing sleeve is shielded from scattering to the image bearing member by the cover member, and is guided and recovered into the developer container.
Under such circumstances, the inventors studied a developing device of multi-stage magnetic brush development type provided with plural developing sleeves, in accordance with the configuration of the conventional technique discussed in the Japanese Patent Application Laid-Open No. 2010-096922. In other words, in a case where a cover member is provided in the outside of the developer bearing region, a sheet supply endurance test for a long period of time was conducted. In this case, a problem as below may occur.
Specifically, a toner drop may occur at an end portion of supplied sheet, during the sheet supply endurance test for a long period of time. Toner drop herein used is a phenomenon in which a toner drops or flies onto originally unintentional area on the sheet, and can end up contaminating an image. Furthermore, when positions at which the toner drops occurred on images were investigated, it was found that the toner drops occurred at an inner side than the cover member in the longitudinal direction, in other words, in the developer bearing region.
Thus, further observations were carried out to identify occurrence locations of the toner drops. Then, it was found that there are toner aggregates flying from longitudinal end portions (outside the developer bearing region, a spacing G2 described in Japanese Patent Application Laid-Open No. 2010-096922) of the spacing between the upstream developing sleeve and the downstream developing sleeve, toward the center of the longitudinal direction. It became clear that such toner aggregates flying toward the center of longitudinal direction, cannot be sufficiently shielded by a cover member discussed in Japanese Patent Application Laid-Open No. 2010-096922, and toner drop images may end up occurring.